The present invention relates to consumable electrode type arc welding apparatuses. More particularly, the invention relates to electrode type arc welding apparatuses in which, while a wire electrode is being fed towards a workpiece to be welded, an arc is generated between the wire electrode and the workpiece to weld the latter.
Heretofore, a consumable electrode type arc welding apparatus of this type is started as follows:
A DC voltage is applied between a wire electrode wound on a reel and a workpiece to be welded, and the wire electrode is fed towards the workpiece by a wire electrode feeding device at a constant speed. When the wire electrode 10 is brought into contact with the surface of the workpiece 12 as shown in FIG. 1(A), a start current flows to the workpiece 12 from the electrode 10. The reason why the start current is caused to flow at the start of the welding operation is that it is necessary to generate a small arc between the end of the wire electrode 10 and the workpiece 12 to melt a certain length of the end portion of the wire electrode and to thereby generate an arc having a desired arc length from the entire end face of the wire electrode 10.
In the conventional arc welding apparatus, the start current rises at a predetermined rate irrespective of welding conditions such as wire diameter and welding current, and therefore the start current is so controlled that a predetermined amount of current is allowed to flow in a predetermined period of time. For instance, as shown in FIG. 2, the current rise curve is so determined that a start current I.sub.1 flows at a time instant T.sub.1 which occurs a predetermined period of time after the time instant T.sub.0 at which the wire electrode 10 is brought into contact with the surface of the workpiece 12. The case where the arc is generated in a relatively short period of time is indicated by the broken line in FIG. 2. In this case, at the time instant T.sub.5 a small arc is generated between the end of the wire electrode 10 and the workpiece 12. As the short-circuited state becomes the arcing state, the current decreases gradually. When the current decreases to a value I.sub.2 at the time instant T.sub.6, the flow of current becomes steady; that is, a welding current I.sub.2 having a predetermined value flows. In the case when the arc is generated in an extremely short time as described above, the arc generating current I.sub.3 is small and therefore very little sputtering occurs at the start of the welding operation.
A specific example of a conventional arc welding apparatus is shown in FIG. 3. In this apparatus, the three-phase alternating current input is subjected to voltage drop by a step-down transformer T.sub.1, the three-phase alternating current output of which is subjected to rectification and phase control by a main circuit thyristor unit to provide a DC output. The DC output thus provided is coupled out of the welding power source through a DC reactor L.sub.2 ; that is, it is supplied between the wire electrode of a welding torch and a workpiece to be welded through a wire feeding device. In this case, the phase control angle (i.e., the DC output value) of the main circuit thyristor unit is determined by a voltage adjustment value set by an operating control head, and the speed of feeding of the wire electrode is determined by a current adjustment value set by the operating control head. The rate of rise of the welding current is determined by the output voltage preset and the inductance of the DC reactor L.sub.2 ; however, only for the start, is the welding control circuit operated to separately determine the output voltage so that it is higher than the ordinary welding voltage. A predetermined period of time after the arc has been generated, an instruction is issued to return the output voltage to the voltage adjustment value set by the operating control head. Thus, at the start of the welding operation, the gradient of rise of the current is completely determined by the output voltage set at the start and the inductance of the DC reactor L.sub.2. The falling waveform of the current is fully determined by the value of the welding current provided when the arc is generated after the wire electrode has contacted the workpiece and by the inductance of the DC reactor L.sub.2, as shown in FIG. 4.
In the conventional apparatus, in order to positively generate an arc, control is so made that a sufficiently long period of time is provided for the initial generation of the arc; that is, an excessively large short-circuiting current flows for the period of time between the time instants T.sub.0 and T.sub.1. Therefore, even when the end of the wire electrode 10 is brought into contact with the surface of the workpiece 12 as shown in FIG. 1(A), the arc will not generate immediately, and instead is generated at the time instant T.sub.1. In this case, the arc is not produced between the workpiece 12 and the end of the wire electrode 10. That is, the current has a considerably large value I.sub.1, as a result of which the wire electrode can be fragmented by a fusing action while producing arcs as shown in FIG. 1(B). In such a case, the wire electrode 10 is burnt away. That is, the conventional apparatus is disadvantageous in that the arcing is not positively started.
Furthermore, in the conventional apparatus, a predetermined amount of short-circuiting current is allowed to flow irrespective of the welding conditions. Therefore, the conventional apparatus suffers from a problem that, when a wire electrode of a different diameter from the one previously employed is used or the welding current value is changed, the arcing will not be positively started. That is, the optimum value of the short-circuiting current is determined by the diameter of the wire electrode, and the start current should be increased with the wire diameter. Similarly, when the welding current is increased, it is necessary to increase the start current. Thus, when generation of the arc is delayed, an excessively large start current increasingly causes sputtering at the start, accelerates the creeping of the arc, and causes a burn back phenomenon frequently whereby the electrode wire 10 is welded to a tip (not shown) used for feeding the electrode wire. In this case also, the arcing cannot be positively started.
In order to overcome this difficulty, a method of controlling the start current according to the welding conditions has been proposed in the art. However, the method is still disadvantageous in that, in a conventional thyristor control system, the start current varies as shown by the characteristic curve of FIG. 2. Therefore, if the peak value of the start current is set to be small, the arc is generated from only a part of the end of the wire electrode. In other words, the arc is not produced from the entire end face of the wire electrode 10. Thus, the produced arc is unstable and is therefore liable to extinction.